Semiconductor device with a cooling element

ABSTRACT

A semiconductor device comprising a semiconductor component, particularly a power laser diode bar, disposed on a cooling element, wherein the cooling element contains in its interior a cooling channel for conducting a coolant. The coolant channel comprises in at least one region microstructures for effective heat transfer to the coolant. The semiconductor component substantially completely overlaps the region of the cooling channel comprising the microstructures. Disposed between the semiconductor component and the cooling element is an intermediate support so arranged and configured that it compensates for mechanical stresses between the semiconductor component and the cooling element occurring as a result of differing thermal expansions of the semiconductor component and the cooling element. The material of the cooling element particularly preferably has a high modulus of elasticity such that the compensation takes place substantially within the elastic strain regime.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/DE2003/001906, filed Jun. 10, 2003, which claims the benefit ofGerman Patent Application Ser. No. 10234704.2, filed on Jul. 30, 2002.The contents of both applications are hereby incorporated by referencein their entireties.

FIELD OF THE INVENTION

The invention concerns a semiconductor device comprising a semiconductorcomponent, particularly a laser diode or a laser bar, disposed on acooling element, wherein the cooling element contains in its interior acooling channel serving to conduct a coolant and comprising in at leastone region microstructures for effective heat transfer to the coolant.

BACKGROUND OF THE INVENTION

A known semiconductor device of this kind is depicted and described, forexample, in patent DE 195 06 093 02. A schematic diagram of such a knownsemiconductor device is provided in FIG. 2. In that case, a microcooler20 is fabricated by bonding together plural copper foils structured byetching. The individual layers form in conjunction a coolant inlet 24; acooling channel 26, which conducts the coolant to the region of themicrocooler 20 on which a power laser bar 12 is mounted; and a coolantoutlet 28. The coolant flows along the arrow 30 from the inlet 24 to theoutlet 28. Implemented in at least one region 32 are microstructures:narrow channels, for example. Particularly effective heat exchange takesplace in that region because of turbulent flow of the coolant.

The laser bar 12 is soldered to the front edge of the microcooler bymeans of a soft solder 52—indium, for example. Mounting the bar 12directly on the copper block 20 improves heat transfer from the laserbar to the cooler.

SUMMARY OF THE INVENTION

A disadvantage with respect to heat transfer to the coolant is the factthat no microstructured area can be placed directly under the laser bar12 because of the seal. A region 54 of heat flow is therefore created,as illustrated in FIG. 2.

As a result of temperature-change loads of the kind that occur, forexample, during cooling after the soldering step or when the laser baris turned on and off, plastic deformation can occur in the softsolder—in the case of indium, for example, in response to temperatureincreases of as little as 5° C. to 7° C. This deformation can cause theconnection to be partially or completely broken. A partially orcompletely interrupted connection delivers much poorer heat dissipationand an undesirable inhomogeneous power distribution in the laser bar.

The object underlying the present invention is, in a device of thespecies cited at the beginning hereof, to improve the thermal andmechanical coupling of the semiconductor component to the microcooler.The invention is particularly intended to provide both high heatdissipation efficiency and high mechanical stability of the arrangement.

This object is achieved by means of a semiconductor device as set forthbelow. Advantageous improvements of the semiconductor device alsofollow.

Although the discussion pertains to power laser bars, it will beunderstood by those skilled in the art that many of the below-describedadvantages of the solution according to the invention do not arisesolely in connection with power laser bars. Rather, the invention canalso be used to advantage with other semiconductor devices in which asemiconductor component is disposed on a microcooler whose thermalexpansion coefficient is different from that of the semiconductormaterial, and which are exposed to substantial temperature changesduring operation.

Apart from the aforesaid power laser bars, this also applies, forexample, to power transistors and power light-emitting diodes, or forexample to semiconductor devices that are used in the automotiveindustry, in aircraft or the like, and are there exposed to majoroutside temperature fluctuations.

According to the invention, it is provided, in a semiconductor device ofthe species cited at the beginning hereof, that disposed between thesemiconductor component and the cooling element is an intermediatesupport that substantially completely overlaps themicrostructure-comprising region of the cooling channel on the coolingelement and that is so arranged and configured that it compensates formechanical stresses occurring between the semiconductor and the coolingelement as a result of temperature differences.

The intermediate support “substantially completely” overlaps the regioncomprising the microstructures as long as the region on which thesemiconductor component is disposed overlaps. It is immaterial in thisregard whether any electrical contact areas or the like project out ofthe overlap area.

The invention is therefore based on the idea of conducting the heat fromthe semiconductor component to the coolant by the shortest possible pathand simultaneously guaranteeing the mechanical stability of theconnection between the semiconductor component and the cooling elementby means of an intermediate support that compensates for differingexpansions.

It is advantageously provided in regard to the semiconductor device ofthe invention that the thermal expansion coefficient of the intermediatesupport is adapted to the thermal expansion coefficient of thesemiconductor component and the intermediate support has a high modulusof elasticity such that said intermediate support compensatessubstantially within the elastic strain regime for mechanical stressesoccurring as a result of temperature differences between thesemiconductor component and the cooling element.

The intermediate support therefore equalizes the differential thermalexpansion of the semiconductor component and the cooling element throughcompletely reversible strain. Mechanical loading of the semiconductorcomponent thus is largely prevented.

The intermediate support preferably has a higher thermal conductivitythan copper, particularly a thermal conductivity that is 1.5 timeshigher, preferably three times higher than that of copper.

In a preferred embodiment of the invention, the semiconductor componentis fastened to the intermediate support by means of a hard solder. Sinceaccording to the invention the intermediate support accommodates thedifferential thermal expansion, this function no longer has to beperformed by a plastically deformable soft solder, as in the prior-artdesigns. This creates the freedom instead to use ahigh-temperature-resistant and cycle-stable hard solder to fastentogether the component and the intermediate support.

In an advantageous improvement of the device according to the invention,the intermediate support is also fastened to the cooling element bymeans of a hard solder, for the same reasons.

In a particularly preferable manner, the intermediate support isfastened both to the cooling element and to the semiconductor componentvia a hard solder or a significantly higher-melting solder than indium.

Preferable candidates for use as hard-solder materials in thisconnection are AuSn, AuGe or AuSi. Higher-melting solders in the abovesense are, for example, SnAgSb, SnCu or SnSb. In the present context,the use of AuSn as hard solder is currently preferred.

In a preferred embodiment of the semiconductor device of the invention,it is provided that the intermediate support is made of molybdenum,tungsten, a copper/molybdenum alloy or a copper/tungsten alloy. Thecopper content of the copper/molybdenum or copper/tungsten alloy isadvantageously between about 10% and about 20%. These materials have ahigh modulus of elasticity of more than 250 GPa or even in excess of 300GPa. They further offer a high yield stress and high temperaturestability.

An intermediate support can be made of these materials not only in theform of foil, but also in the form of a layer that is sputter-deposited,vapor-deposited or galvanized onto the cooling element. It is understoodthat in the latter cases there is no need to fasten the intermediatesupport to the heat sink by hard soldering.

In another preferred embodiment of the invention, the intermediatesupport comprises a diamond composite material, particularly adiamond/metal matrix material. An intermediate support of this kindpreferably contains at least one of the material combinationsdiamond/copper, diamond/cobalt and diamond/aluminum. These materialsoffer a higher thermal conductivity than copper—up to 600W/mK—accompanied by expansion coefficients that are roughly the same asthose of the semiconductor component. With the use of a copper/diamondintermediate support, the connecting layer to the semiconductorcomponent preferably contains AuSn and the connecting layer to thecooling element preferably contains SnAgSb.

In a particularly preferred manner, the application of the structure ofthe invention is power semiconductor laser diode bars, particularlyAlGaAs-based such bars.

In a preferred improvement of the semiconductor device according to theinvention, a laser diode and a beam-collimating device, preferably amicrolens, are disposed on one and the same outer surface of the coolingelement. The beam-collimating device collimates the beam divergence ofthe laser diode. Without the beam-collimating device, the intermediatesupport can be shifted backward by no more than tan(beam divergence)without eliciting shadowing by the cooling element. Normally this is notsufficient for the bar plus the intermediate support to be positionedcentrally enough to the microcooling structures.

A microlens for beam collimation is frequently also used in conventionaldevices of prior art. As shown in FIG. 2, owing to the conventionalarrangement of the laser bar 12 at the edge of the microcooler, themicrolens 62 is attached to the cooler 20 by means of an auxiliarymounting 60.

By contrast, no corresponding extra attaching part is needed with thedevice according to the invention.

In a preferred embodiment of the semiconductor device according to theinvention, it is provided that the cooling element comprises pluralstacked, areally interconnected layers, a portion thereof beingstructured, to form in the interior of the cooling element the coolingchannels for conducting the coolant.

These layers of the cooling element are preferably formed of copperfoils structured by etching.

Further advantages, advantageous embodiments, features and details ofthe invention will emerge from the following description of anembodiment example with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Only the elements essential for an understanding of the invention areillustrated in the drawings. Therein:

FIG. 1 is a schematic diagram of a sectional view of the embodimentexample; and

FIG. 2 is a schematic diagram of a sectional view of a semiconductordevice according to the prior art (described in more detail earlierhereinabove).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The semiconductor device 10 illustrated in section in FIG. 1 comprises apower laser diode bar 12, which is soldered onto a microooler 20 havinga coolant inlet 24 on its upper face, a cooling channel 26 in itsinterior and a coolant outlet 28 on its bottom face. The direction offlow of the coolant in the microcooler 20 is indicated by arrows 30.

Cooling channel 26 comprises, in a region 32 beneath power laser diodebar 12, microstructures, for example a plurality of channels eachmeasuring 0.3 mm in width and height. Such microstructures causeturbulence in the flowing coolant by means of which the exchange of heatbetween the coolant and the microcooler can be configured veryeffectively.

The length of the microstructured region 32 in cooling channel 26 is atleast equal to the length of power laser diode bar 12, which completelyoverlaps microstructured region 32 at least in this direction of extent.Preferably, as illustrated in FIG. 1, the length of microstructuredregion 32 is greater than that of power laser diode bar 12, therebyenlarging the cross section of the region in which the flow of heat frompower laser diode bar 12 to microstructured region 32 takes place.

In a particularly preferred manner, microstructured region 32 is alsoexactly as wide as or wider than power laser diode bar 12.

Disposed between power laser diode bar 12 and microcooler 20 is anintermediate support 16, which is soldered onto the surface of andcompletely overlaps microcooler 20 above microstructured region 32.

Intermediate support 16 is composed, for example, of a copper/tungstenalloy with a copper content of 15%, and has a thickness of, for example,250 μm.

The connection between power laser diode bar 12 and intermediate support16 and the connection between intermediate support 16 and the surface 22of microcooler 20 is made with AuSn, a solder that has substantially noplastic properties. These hard-solder layers are denoted in FIG. 1 byreference numerals 14 and 18.

Owing to its high modulus of elasticity, intermediate support 16accommodates mechanical stresses occurring for example due tooperation-induced heating and differential thermal expansion of thematerials of power laser diode bar 12 and microcooler 20 (copper) withinthe elastic strain regime, so that the risk of damage to hard-solderlayers 14, 18 and/or power laser diode bar 12 is reduced to the greatestpossible extent.

Intermediate support 16 does have a lower thermal conductivity than apure-indium soft-solder connection between the power laser diode bar andthe microcooler, of the kind known in the prior art. However, thisreduced thermal conductivity is more than offset by the much morefavorable heat transfer from the power laser diode bar 12 to themicrostructured region 32, so that, in the aggregate, improved heat flowis achieved between the power laser diode bar and the coolant comparedto the conventional design illustrated in FIG. 2.

With the design illustrated in FIG. 1, the thermal resistance R_(th) fora power laser diode bar 10 mm long is up to 40% lower than the valuesattained with conventional designs.

As shown in FIG. 1, a microlens 40 for beam collimation is disposed onthe surface of micro-cooler 20 on which power laser diode bar 12 ismounted. Surface 22 advantageously offers a suitable mounting surfacefor this purpose in the immediate vicinity of the power laser diode bar.Auxiliary parts or add-ons to the microcooler of the kind necessary withknown devices are not required with a device making use of the technicalteaching disclosed in the foregoing.

Naturally, the explanation of the invention based on the embodimentexample is not to be construed as limiting the invention thereto.Rather, the features of the invention disclosed in the preceding generalpart of the description, in the drawing and in the claims are essentialto the practice of the invention both individually and in anycombination that may appear suitable to one skilled in the art. Forexample, instead of the intermediate support 16 made of copper/tungstenalloy that is cited exemplarily in the embodiment example, which ispreferably connected on its two sides respectively to cooling element 20and to semiconductor component 12 by means of respective hard solders14, 18, an intermediate support 16 comprising a diamond compositematerial can be used, as set out in the general part of the description.If a copper/diamond intermediate support is used, connecting layer 14 tosemiconductor component 12 preferably contains AuSn and connecting layer18 to cooling element 20 preferably contains SnAgSb.

1. A semiconductor device comprising a semiconductor component comprising a power laser diode bar, disposed on a cooling element; said cooling element containing in its interior a cooling channel serving to conduct a coolant and comprising in at least one region microstructures for effective heat transfer to said coolant, wherein said semiconductor component substantially completely overlaps said region of said cooling channel comprising said microstructures, and disposed between said semiconductor component and said cooling element is an intermediate support so arranged and configured that it compensates for mechanical stresses between said semiconductor component and said cooling element occurring as a result of differing thermal expansions of said semiconductor component and said cooling element; and a beam-collimating device, wherein the laser diode bar and the beam-collimating device are disposed on a common surface of the cooling element.
 2. The semiconductor device as set forth in claim 1, wherein said intermediate support has a high modulus of elasticity such that it compensates for the mechanical stresses substantially within the elastic strain regime.
 3. The semiconductor device as set forth in claim 1, wherein said intermediate support has a higher thermal conductivity than copper, particularly a thermal conductivity that is about 1.5 times higher than that of copper.
 4. The semiconductor device as set forth in claim 1, wherein the thermal expansion coefficient of said intermediate support is adapted to the thermal expansion coefficient of said semiconductor component.
 5. The semiconductor device as set forth in claim 1, wherein said semiconductor component is connected by means of a hard solder to said intermediate support.
 6. The semiconductor device as set forth in claim 1, wherein said intermediate support is connected by means of a hard solder to said cooling element.
 7. The semiconductor device as set forth in claim 5, wherein the hard solder comprises a AuSn-based solder material.
 8. The semiconductor device as set forth in claim 1, wherein said intermediate support is fabricated of molybdenum, tungsten, a copper/molybdenum alloy or a copper/tungsten alloy, preferably having a copper content of about 10 to about 20%.
 9. The semiconductor device as set forth in claim 1, wherein said intermediate support comprises a diamond composite material, particularly a diamond/metal matrix material, which particularly contains at least one of the material combinations diamond/copper, diamond/cobalt and diamond/aluminum.
 10. The semiconductor device as set forth in claim 1, wherein said cooling element is composed of plural stacked, areally interconnected layers, a portion thereof being structured, to form in the interior of said cooling element said cooling channel for conducting said coolant.
 11. The semiconductor device as set forth in claim 10, wherein the layers of said cooling element are formed at least in part by the etching of structured copper foils.
 12. The semiconductor device as set forth in claim 1, wherein the length of the micro structured region is at least equal to or greater than the length of said semiconductor component and said microstructured region completely overlaps said semiconductor component in the lengthwise direction.
 13. The semiconductor device as set forth in claim 1, wherein the width of said microstructured region is equal to or greater than the width of said semiconductor component and said microstructured region completely overlaps said semiconductor component in the widthwise direction.
 14. The semiconductor device as set forth in claim 1, wherein the beam-collimating device comprises a microlens.
 15. A semiconductor device, comprising: a semiconductor component disposed on a cooling element, the cooling element comprising an interior cooling channel configured to conduct coolant, and comprising microstructures configured to transfer heat to the coolant; and an intermediate support disposed between the semiconductor component and the cooling element, the intermediate support being configured to compensate for mechanical stresses between the semiconductor component and the cooling element as a result of differing thermal expansions of the semiconductor component and the cooling element, wherein the semiconductor component substantially completely overlaps a region of the cooling channel that comprises the microstructures, and the intermediate support is formed of a diamond/metal matrix material that comprises at least one metal selected from the group consisting of copper, cobalt, and aluminum.
 16. The semiconductor device as set forth in claim 6, wherein the hard solder comprises a AuSn-based solder material. 